This invention relates to a jack-up platform locking apparatus. More specifically this invention relates to an apparatus for locking an offshore platform deck with respect to a vertically adjustable leg of the platform independently of a platform jack-up drive assembly.
In the past, offshore platforms or towers have been extensively utilized around and upon the continental shelf regions of the world. Examples of offshore platform installations include supports for radar stations, light beacons, scientific and exploration laboratories, chemical plants, power generating plants, etc. Principally, however, offshore platforms have been utilized by the oil and gas industry in connection with oil and gas drilling, production and/or distribution operations.
In conducting such offshore activity several platform designs have been utilized in the industry. In deep water applications, semi-submersible or floating drill ships which are dynamically positioned and/or turret moored over a well site have been effectively utilized. In shallower water applications fixed length towers have been fabricated on shore and either transported in a horizontal posture to an offshore site by barge or towed upon bouyancy chambers within the tower legs. On site the tower is pivoted into an upright posture and the base of the tower legs are positioned into firm engagement with the seabed. A platform deck is then fabricated upon the erected tower for conducting offshore operations. Such fixed platforms require considerable time to assemble and once in position are difficult to relocate.
One platform design which combines many of the advantages of floating and fixed equipment is known as a "jack-up platform". In this connection a barge or self-propelled deck, operable to function in a conventional flotation capacity during transportation, is fitted with extendable legs which are deployable on site. More specifically, the hull/deck carrying one or more jack-up legs is either towed or navigated to a desired offshore site and the legs are jacked through wells in the deck into firm supporting engagement with the water bed. Further jacking serves to raise the deck with respect to the surface of the body of water and once the lowermost portion of the deck is positioned above a statistical wavecrest height jacking is discontinued and drilling and/or production operations are begun.
The subject invention is directed to a deck locking apparatus for a jack-up platform such as previously described.
Various designs have been advantageously utilized to jack the supporting legs with respect to the platform deck. One such design comprises a jacking mechanism having hydraulic cylinders which reciprocate vertically along the legs of the offshore tower and carry horizontally actuatable pins at the ends thereof for engagement with apertures positioned within rails affixed to the legs. Another previously known jacking mechanism comprises one or more racks which longitudinally extend along the operative jacking length of the offshore tower legs. Pinion gears mesh with the leg mounted racks and are driven by hydraulic or electric drive assemblies mounted upon the platform deck.
Although jack-up platform designs have attained a considerable degree of commercial acceptance, at least some jack-up mechanisms, and particularly the jack-up mechanisms of the rack and pinion type, permit a degree of undesireable slack or vertical mobility between the legs and platform. Such mobility may prove to be disadvantageous when the platform encounters environmental hydrodynamic and aerodynamic loading. More specifically platform legs typically comprise a set of three or more vertical chords which are interconnected and strengthened with "K" or "X" type bracing. As previously mentioned the legs extend through wells fashioned within the platform deck and peripherally pass adjacent bearing surfaces at the upper and lower surface of the deck. Tolerance within some jack-up mechanisms, combined with bending moments encountered from environmental loading, tends to displace the legs into abutting lateral contact with the upper and lower surfaces of the deck. Accordingly typical platform legs must be overdesigned to carry lateral reaction forces at locations intermediate the junctions of the "K" or "X" type bracing.
In a similar vein present jack-up systems are fabricated with large structural members in order to withstand and react against environmentally imposed moments. The structural size of these members needed to safely react peak statistical moments adds a considerable degree of dead weight and cost to the overall platform design.
The difficulties suggested in the preceding are not intended to be exhaustive, but rather among many which may tend to reduce the effectiveness and owner satisfaction of prior jack-up offshore platform systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that jack-up platform systems particularly of the rack and pinion type appearing in the past will admit to worth-while improvement.